This invention relates to a compact electronic device having a key switch structure and, more particularly, to an improvement in an anisotropically electrical key switch structure suitable for compact electronic equipment.
Conventionally, a contact type key switch is adopted as a key switch in electronic equipment such as a compact electronic calculator. A contact type key switch consists of a stationary contact and a movable contact opposing the stationary contact at a given interval and contacting the stationary contact upon depression. Recently, an anisotropical key switch has been proposed. In an anisotropical key switch, an anisotropically electrical conductive rubber sheet is provided on a key contact corresponding to the stationary contact of the contact type key switch and a conductor corresponding to the movable contact thereof is provided on the conductive rubber sheet.
The anisotropically electrical key switch exhibits an insulative property when no pressure is applied thereto, and when a compression force is applied, the compressed portion exhibits conductivity. By utilizing such characteristics of an anisotropically electrical conductive rubber sheet, a conductive layer portion is compressed so as to turn on a switch. In the anisotropically electrical switch of this type, since a gap between the stationary and movable contacts need not be maintained unlike in a contact type key switch, the arrangement of the key switch section of compact electronic equipment can be simplified by utilizing the anisotropically electrical rubber sheet.
However, the anisotropically electrical key switch structure of conventional compact electronic equipment is as follows. A key contact is formed on an upper surface of a printed circuit board on which a predetermined circuit pattern is formed, an anisotropically electrical conductive rubber sheet is overlaid on the key contact, and a conductor opposing the key contact is provided thereon. When the electronic equipment is manufactured, the anisotropically electrical conductive rubber sheet must be aligned and fixed on the printed circuit board. For this reason, assembly of the key switch is cumbersome. When the anisotropically electrical conductive rubber sheet is formed too thin, special handling of the rubber sheet is required when it is fixed on the circuit board, to prevent the rubber sheet from forming wrinkles and being damaged. In order to overcome this drawback, the rubber sheet is formed to be thick (normally 0.5 to 0.1 mm) so as to assure mechanical strength. However, in this case, the key switch section becomes too thick and the equipment cannot be formed to be thin. Although current compact electronic equipment utilizing a contact type key switch is very thin, e.g., of a total thickness of 1 mm or less, if an anisotropically electrical key switch is used and the anisotropically electrical conductive rubber sheet is formed to be 0.5 to 1.0 mm in order to obtain a satisfactory mechanical strength, the total thickness of the equipment becomes too large.